I think Russia is currently engaged in a European, 2022 version of the Vietnam war. It's bound to go about as well as that went for the US. I am honestly a bit surprised that someone like Putin engaged himself in something like this, but maybe there's a long game that we still have to see. Even setting aside everything else I don't understand what Russia could possibly have to gain from this.
Here's my take informed by Guardian analysts combined with ignorance of the details.
1. Putin decided already in 2014 that he can't get Ukraine back to the Russian fold by asking nicely, so he decided to take from it what he could: Crimea and the Russian separatist states.
2. Putin is semi-retiring in 2024 and thinks about his legacy to Russia. He decided he wants to end the Ukrainian standoff by force.
3. He predicted he would bring an overwhelming force to secure a quick victory and demilitarize Ukraine by destroying its military equipment after they surrender and that Ukraine would be forced to accept harsh terms about limiting their military size and keeping away from NATO and EU. This is a very similar scenario to Georgia 2008. He would also expand the borders of the Russian entities to take much more territory.
His army would retreat from Ukraine once this is done, there's no reason to stay there after they get what they want, they don't want to fight insurgents and they don't even need to install puppets. They do need to protect the separatist republics though, so some army units may stay there.
4. He didn't count on Ukraine's resolute defense and the willingness of the EU to inflict pain on Russia at cost to themselves, which was again inspired by Ukrainian's spirit of resistance and the leadership of their president.
5. He may still win and do his Georgia thing, but he may also get stuck in the big cities and be bled to death (at the cost of Ukrainian civilian lives, because he will use artillery in urban areas if he's stuck there).